1. Field of the Invention
The present application relates generally to systems and methods for determining the depth, the velocity, or both the depth and the velocity of an instrumentation package within a wellbore.
2. Description of the Related Art
Surface-based wellbore depth measurements are typically made periodically during wellbore drilling for the exploration of oil and gas deposits to determine the absolute depth of the drilling tool within the wellbore. In such measurements, the depth of the drilling tool is typically determined by surface measurements of the lengths of the pipe sections inserted into the wellbore between the drilling tool and the surface.
Using wireline surveys, the drilling of the wellbore is periodically halted and a survey tool is lowered into the wellbore. As the survey tool is guided along the wellbore, it can provide information regarding its orientation and location by sending signals through a wire or cable to the surface. The absolute depth of the survey tool down the wellbore is typically given by a surface-based measurement of the length of the wire or cable between the survey tool and the surface. Similarly, surface-based logging measurements of the absolute depth of detected geological formations are typically made by surface measurements of the length of the wire or cable between the logging tool and the surface. However, due to various distortions, the cumulative length of the pipe sections or of the wire while within the wellbore can differ from the cumulative length measured at the surface, resulting in errors in the determination of the absolute depth.
In addition, surface-based measurements of the velocity of the survey tool along the wellbore can be used to determine the relative distances between detected features or formations within the wellbore. For example, the time period between detecting two separate features along the wellbore and the velocity of the survey tool during this time period can be multiplied together to provide the relative distance between the two detected features. However, as with the surface-based depth measurements described above, surface-based velocity measurements do not provide a sufficient accuracy (e.g., within only a few centimeters) to tell when two geophysical sensors with significant along-hole separation pass the same geological formation within only a few centimeters of accuracy. For example, due to friction or other effects within the wellbore, the survey tool can move in a jerking manner with varying velocity. In addition, these effects can result in the velocity of the survey tool within the wellbore differing from the measured velocity of the wire or cable at the surface.
Inertial navigation systems have been proposed as being able to provide improved wellbore depth and velocity measurements. Such inertial navigation systems can determine the depth of the survey tool by double integration of the detected acceleration. However, such procedures are vulnerable to errors in the detected acceleration, which results in a drift of the depth measurement from the true depth of the survey tool. It has been difficult, and will likely continue to be difficult, to get such inertial navigation systems to work without providing aiding data such as surface-based depth measurements or updates of the depth obtained while the survey tool is stationary (i.e., zero-velocity updates) to remove this drift. However, it is generally desirable to avoid using surface-based aiding data, since such data would effectively transform an inertial navigation system into a surface-depth system, thereby including the problems of such systems. It is also generally desirable to avoid zero-velocity updates due to large increases in the time and costs of conducting such surveys.
Improved wellbore depth and velocity measurements are desirable to understand the geological formations being drilled and the oil or gas deposits being accessed. For example, improved measurements can resolve uncertainties in the depth measurements of a geological fault from wellbore positioning surveys in two nearby wells. In addition, improved wellbore depth measurements are helpful for drilling safety by providing more reliable information regarding the true wellbore depth to avoid drilling into adjacent wells.